The present invention relates to a method of producing a silicon carbide MOSFETs by using silicon carbide (SIC) as its main material.
Conventionally, a silicon single crystal has been used as the material for a power semiconductor device for controlling a large current and a high withstanding voltage. There are various kinds of power semiconductor devices and they are selectively used in accordance with a use in the existing circumstances. For example, in the case of a bipolar transistor, high speed switching cannot be performed although it is large in current density, and therefore it has a limit in use at several kHz. A power MOSFET, on the contrary, can be used at a high speed up to several MHz although it cannot treat a large current. Since power devices having large current and high speed properties have been earnestly required in a market, however, efforts have been given on the improvement of bipolar transistors, power MOSFETs, and so on, and the development has advanced near a limit in view of materials. Material examination has been performed in view of power semiconductor devices has been performed and it is considered that GaAs, diamond, and SiC have a large advantage as materials as reported by Beliga in IEEE Electron Device Letters, Vol. 10 (1989), p. 455 and by Shenai et al. in IEEE Transactions on Electron Devices, Vol. 36 (1989), p. 181. In the case of GaAs, however, it is difficult to apply GaAs to a gate drive device represented by an MOS because such a high-quality insulating film as silicon cannot be obtained by GaAs although GaAs has been applied to Schottky diodes. Further in the case of diamond, it is impossible to artificially produce a large diameter single crystal from diamond, it is difficult to control the conductivity type, and it is difficult to use diamond in the form of a semiconductor. In the case of SiC, on the other hand, a single crystal can be formed and wafers having a diameter of 1 inch are available on the market, the diameter thereof being shifted to 2 inches. Further, SiC is more advantageous than the other materials in the fact that the conductivity type can be controlled and SiO.sub.2 acting as an insulating film can be grown by thermal oxidation similarly to the case of silicon. In these points of view, trial manufacture of transistors such as MOSFETs and so on have been reported and the MOS operation has been confirmed by Palmour et al. in J. Appl. Phys. Vol. 64 (1988), p. 2168 and by Davis et al. in Proceedings of the IEEE, Vol. 79 (1991), p. 677. All the MOSFETs which have been generally used are of the type in which a current is made to flow laterally, and, therefore, they cannot be applied as they are to power semiconductor devices requiring a large current. A vertical power MOSFET using silicon has a configuration in which, as shown in FIG. 2, a p-type base region 22 is formed in a surface layer of an n-type base layer 21 and a gate electrode 26 is formed on the surface of the n-type base layer 21 through a gate insulating film 25 so as to form an n-type channel in a portion 24 between a source region 23 and an exposed portion of the n-type base layer 21, so that a current is made to flow from a source electrode 27 commonly contacting with the n.sup.+ -type source region 23 and the p-type base region 22 into a drain electrode 29 contacting with an n.sup.+ -type buffer layer 28 at the rear surface side of the n-type layer 21. Thus, it is deviced that a current is made to flow vertically by use of the whole chip surface. In this MOSFET, the n-type channel is formed in the surface region 24 by application of a voltage to the gate electrode 26, and the source electrode 27 and the drain electrode 29 are conducted to each other. In this MOSFET, it is possible to inhibit a high voltage by increasing the thickness of the n-type base layer 21 which is one of junction layers between the p-type base region 22 supplied with a reverse voltage and the n-type base layer 21.
There is a significant problem in application of such a configuration as it is as shown in FIG. 2 to an SiC device. SiC is advantageous in that it has a stable chemical property and the strength of combination between crystals is stronger than that of Si, and, therefore, adversely, diffusion of impurity is hardly generated. That is, it is very difficult in the case of SiC to generate impurity diffusion of donors and acceptors, which is a fundamental technique with respect to Si,and the fact that diffusion is hardly found even at 1700.degree. C. has been reported by Addamiano et al. in Journal of the Electrochemical Society, Vol. 119 (1972), p. 1355 and by Gusev et al. in Sov. Phys. Semicond., Vol. 9 (1976), p. 820.
In the case of producing the Si power MOSFET of FIG. 2, the p-type base region 22 is formed in such a manner that impurity ions are injected by using the gate electrode 26 as a mask and diffused through high-temperature heat treatment. This technique called self alignment double diffusion is important in formation of a high-quality device. But, this technique cannot be applied as it is to SiC devices because of the foregoing reasons.